In general, the present invention relates to electrical control systems in which the power consumed by a plurality of loads connected to a power source is monitored and compared with a reference input. The reference input is selected to effect a predetermined constant power rate or a variable rate that over a given interval yields a desired total power usage. If the comparison with the reference input indicates an overload condition (excessive power rate) then one or more of the loads are automatically disconnected (shed). If an underload condition (inefficiently low power rate) is indicated, one or more loads are automatically connected to the source (added). The add and shed load corrections adjust the actual rate of consumption so that it falls within an acceptable range centered about the reference rate.
Systems incorporating the principles of the present invention, as more fully described herein, can be adapted for use in a wide variety of applications. However the invention is primarily intended for use in a power monitoring and limiting system of the type commonly employed by customers of electrical utilities for enabling the customer to obtain the most favorable billing rate offered by the utility. There are a number of variations of such systems, differing in the manner in which reference input is selected and in some cases varied to yield a desired power usage, and some of the more common types are rate systems, ideal curve systems and forecasting systems.
The benefits derived from power monitoring and limiting systems are best understood by reference to the operation of the utility company. The efficiency of the company depends to a large extent on the uniformity and consistency at which power flows from the central station, through the distribution networks, to the numerous utility users. In order to achieve and maintain a high level of efficiency, utilities have adopted certain practices designed to encourage customers to even-out their power consumption and in particular to avoid temporary, extraordinary peak loads that are incapable of being efficiently met by the utility's available power resources.
One practice in this respect is to bill the customer according to a formula that includes both the total energy consumed by the customer over a billing period, such as one month, and the highest or maximum rate of power consumed during any one of a continuous succession of relatively short time intervals, known as demand intervals. Typically, each demand interval is either 15, 30 or 60 minutes. Penalty charges are levied for overload conditions that use an abnormally high amount of power during any given demand interval. Thus it is to the benefit of the utility customer to even-out the rate of power consumption, by limiting the number of connected loads, as necessary, to prevent costly overload conditions.
For this purpose, power monitoring and limiting systems have been devised for monitoring the rate of power consumption and automatically shedding loads as necessary to avoid or minimize overload conditions, and automatically adding loads when needed to bring the power rate up to a level at which the electrical facilities of the user are at optimum utilization.
The present invention concerns improvements in method and apparatus for automatically performing the adding and shedding of electrical loads in systems such as described above. In particular, it is an object of the present invention to provide digital load control circuitry of the type that is responsive to load add and shed command signals in which provision is made for efficiently and precisely controlling a relatively large number of loads and for flexibility in the selection of the order or orders in which the loads are to be added and shed. The latter capability is of advantage in tailoring the control system to each user's electrical installation and in meeting changing conditions or requirements of each installation.
Another object of the present invention is to provide such load control circuitry having the capability of controlling one group of loads out of the total in accordance with a fixed priority add-shed schedule and for controlling another group of such loads in a non-priority rotated add-shed sequence. In this regard, one embodiment of the present invention provides digital circuitry that enables one group of loads to be selected as a fixed priority load group, within which the loads are added in a fixed sequence and shed in the reverse of such sequence, and enables another group of loads to be selected as a rotate load group, in which the loads thereof are added and shed in a variable rotating order. The add/shed order of the rotate group per se, is rotated in a manner similar to that disclosed in U.S. Patent application Ser. No. 587,150 for METHOD AND APPARATUS FOR CONTROLLING ELECTRICAL LOADS, filed June 16, 1975, and now U.S. Pat. No. 4,031,406 and assigned to the assignee of the present application.
A still further object of the present invention is to provide method and apparatus of the above type in which each of a plurality of loads can be quickly and easily selected to be a member of the fixed priority group or of the rotate group by means such as selector switches, to thereby afford substantial flexibility both in the initial set-up and in any subsequent alteration required of the add-shed sequences.
A further object of the present invention is to provide method and apparatus of the above type having the capability of selecting minimum and/or maximum limits on the number of rotate group loads (out of a total number of rotate loads) that can be turned OFF in response to the automatically produced add and shed command signals. In particular, the maximum limit enables the system to shed a succession of loads in response to a series of shed commands until the preset maximum number of OFF loads is reached, whereupon further shedding of rotate loads is prevented, thereby insuring minimal but safe lighting, heating, etc. Similarly, the minimum limit restricts the number of loads that can be automatically turned ON by a series of add commands and serves as an energy saving feature.
Additionally, it is an object of the present invention to provide method and apparatus of the above type that has the capability, in a standard production unit, of accommodating a variable number of controlled loads, so that a user of such unit can either increase or decrease his load capacity as needed to meet changing conditions.